There are two imaging technologies, 1) Reduced field of View Imaging and 2) Parallel Imaging, which go under several names, the two most fundamental are SENSE and SMASH. When a factor of two in time reduction is required, all of the above approaches use the same k-space sampling pattern, i.e. every other line of k-space is sampled.
The Reduced Field of View approach assumes that only a part of the field of view is dynamic, and that the fold over information can be removed by subtracting that information from a previously acquired fully resolved image.
The SMASH and SENSE approaches are related in that they use the sensitivity profiles of separate receiver coils to remove the folded over data. They rely on using two or more receiver coils, each with a distinctly different sensitivity profile to the body section being imaged. Essentially, the folded over data are removed for images in pairs (or higher combinations) of coils with each image pair having different sensitivity characteristics as determined by the coils. The SMASH approach performs this disentangling operation in the k-space domain, while the SENSE approach performs this operation in the image domain.